The present invention pertains generally to light wavefront detection devices and their methods of use. More particularly, the present invention pertains to wavefront detection devices that measure deviations between the actual focal points of individual light beams in the wavefront and the idealized focal point of corresponding individual light beams in a plane wavefront. The present invention is particularly, but not exclusively useful for using light beam focal point deviations for the purposes of restructuring, mimicking or otherwise detecting a light wavefront.
Wavefront detectors are typically used as measurement systems for detecting the shape of the wavefront that is characterized by a plurality of aberrated light beams. To do this, the phase information of the aberrated light beams, i.e. wavefront, can be described by comparing this information to the phase information of an ideal wavefront which is sometimes called the reference wavefront. For this purpose, the reference wavefront is assumed to have its vertex parallel to the optical axis for each point in a plane. The optical path difference (OPD) between the phase information of the aberrated wavefront and the phase information of the ideal reference wavefront is thus quantified at each point of the reference wavefront. The result is that the wavefront of the aberrated light beams will correspond to the sum of all optical aberrations introduced into this beam while travelling through different optical elements within the optical path. Due to the precise nature of their measurements, wavefront detectors have found numerous applications in areas of optical information processing, especially astronomical research, quality control for optical elements, and ophthalmologic diagnostics.
In most wavefront detection schemes interferometers are used for precise and reliable measurements of optical path differences. An accuracy of several fractions of the used radiation wavelength is achievable using interferometer techniques and this accuracy allows high resolutions in quantification of the wavefront""s shape. Different types of interferometers have been invented for this purpose, with the most important being commonly known as the Michelson interferometer. Special redesigns of the Michelson interferometer, however, have been adapted to special measuring purposes. For example, the Mach-Zehnder interferometer or the Saganc interferometer have been widely used. It is common to all types of interferometers that they consist of a system of two optical paths. One path guides a reference wavefront, and the other path contains an element with the optical aberrations that are to be measured. The superposition of the reference wavefront and the aberrated wavefront in an imaging plane then allows the quantification of the amount of aberrations with high accuracy. The disadvantage of this method, however, is the need for two optical paths.
In addition to interferometers, two other type wavefront detection schemes are now offering the possibility of measuring the wavefront""s shape by examining images of a single aberrated light beam. One such method uses a back-projection of the point-spread-function (PSF) of the focused light beam to calculate the light beams wavefront before being focused. In this measuring scheme, the intensity distribution of a focal point generated by a lens is examined by use of a CCD camera and a digital signal processing system. By reconstructing the incident light beams wavefront in front of the lens and comparing it to the focal points PSF, the wavefronts shape can be derived.
The second commonly applied detection scheme with image examination is referred to as the Hartmann-Shack detection scheme. For this scheme, the Hartmann-Shack-wavefront sensors use an array of micro-lenses to divide the incident light beam into a matrix of sub-apertures. Each lens then focuses its partial incident light into a focal point. In case of a local tilt of the incident light beams wavefront within the margins of this sub-aperture, the focal point emerges at a deviation perpendicular to the optical axis. The amount of this deviation, in first order, is proportional to the amount of the local tilt of the wavefront. Thus, this tilt can be quantified. A measurement of all focal point deviations in a wavefront allows the reconstruction of the global wavefront""s shape by use of a least-square-fit method for calculation. This results in a mathematical standard description of the OPD with respect to a reference wavefront by use of high order polynomials.
The basic Hartmann test has been commonly used to measure the surface quality of primary mirrors in astronomical telescopes as they are polished. To do this, a pinhole is placed in the entrance pupil of a lens of high quality. The pinhole is then movable perpendicular to the optical axis to cover every point within the apertures area. Consequently, the wavefront of the incident light beam is divided into a number of sampling points. For each point, the image of the pinhole will result in a focal point on the image plane of the lens and a local tilt of the wavefront within the pinhole will cause a deviation of the focal point perpendicular to the optical axis. As indicated above, this deviation is measurable.
To overcome the critical time limitations of the former described Hartmann-test, a parallel use was invented by Dr. Roland Shack of the University of Arizona""s Optical Sciences Center which is now widely referred to as a Hartmann-Shack wavefront detection scheme. Instead of only one pinhole, a number of equidistant micro lenses is used to generate a matrix of focal points on an image plane.
A Hartmann-Shack-Sensor (HSS) commonly includes an optical system for imaging the aberrated light beam onto a lens array, and an image detector for measuring deviations of the resultant focal points. When using micro-lens array having sufficient quality, HSSs can be used in many applications of wavefront measurement. Further, apart from monitoring for quality control of optical elements where only measurements of wavefront shapes are performed, active optics which use HSS are also realizable. For instance, astronomical telescopes usually compensate for atmospherical distortions by using a closed loop active optics which include a Hartmann-Shack-Sensor.
The critical function of an HSS as part of an image system is to measure the amount of the focal points deviations with a sufficient repetition rate and acceptable accuracy. In operation, the image data is transmitted to an image processing system with digital data acquisition possibilities that are able to perform pattern recognition. The matrix of focal points that needs to be examined for intensity distribution and for the center of each focal point, in order to measure the amount of focal point deviation due to a local wavefront tilt, is often extensive. In this regard, standard CCD (charge coupled devices)-cameras are restricted to a frame repetition rate of about 50 Hz Some high performance CCD-systems, however, with the capability of reading out randomly accessible imaging detectors, are capable of repetition rates of several hundred Hz. Nevertheless, even more responsive and more accurate focal point detections are desirable.
The crucial part of focal point deviation detection is the classification of the focal points intensity distribution for the center of each spot. It is commonly known, that the center of the spots are best derived by use of a center of gravity algorithm. This algorithm achieves a high accuracy in position detection despite the cost of the duration of calculation.
A device for detecting a wavefront in accordance with the present invention includes an optical means, such as a lenslet array, for separating the wavefront into a plurality of contiguous light beams. The individual light beams are then individually focused by the optical means to a plurality of respective focal points in an x-y plane. As contemplated by the present invention, the optical means that is used for separating the wavefront into the plurality of contiguous light beams, and for then focusing these light beams, can be of a type commonly referred to as a Hartmann-Shack-Sensor (HSS).
For the present invention, an array of clusters are positioned to lie in the x-y plane so that the focal point of each individual light beam will be incident on a respective cluster. More specifically, the clusters are contiguously arranged in rows and columns in the x-y plane. Further, each cluster includes a plurality of pixels which are also aligned in rows and columns. In particular, each row in the cluster has a plurality of pixels that are aligned in the x-direction, and each column has a plurality of pixels that are aligned in the y-direction. Still further, each pixel includes a first photodetector (photocell) that will generate an x-signal in response to an illumination of the pixel by the focal point of a light beam, and each pixel includes a second photodetector (photocell) that will generate a y-signal in response to an illumination of the pixel.
Depending on the size of a light beam""s focal point, and the number and size of the pixels that are in a cluster, the actual location of a focal point on the cluster can be determined with good accuracy. For most applications, it is necessary that only about five percent of the pixels in a cluster be illuminated by the focal point of a light beam at any point in time. This is generally sufficient for determining the x-y position of the focal point on the cluster and further, depending upon the location of the particular cluster in the array, the x-y position of a particular focal point can be determined relative to the x-y positions of all other focal points of light beams in the wavefront.
As contemplated by the present invention, the two photodetectors (photocells) which make up each pixel can be either a photodiode type photocell or a phototransistor type photocell. As is well known, various structural configurations are possible for the manufacture of the photodiodes, or the phototransistors, and the selection of a particular type photocell will, for the most part, depend on the particular application that is to be accomplished by the device, and on the wavelengths of the light beams that are to be detected by the device.
In the operation of the device of the present invention, each of the plurality of contiguous light beams that make up the wavefront to be detected is focused onto a respective cluster in the array. The respective xe2x80x9cxxe2x80x9d and xe2x80x9cyxe2x80x9d signals thereby generated are first converted into compressed digital signals. These digital signals are then sent to a computer where they are used to determine the actual x-y position of the particular light beam""s focal point on the cluster. Similarly, the actual x-y positions of the focal points for all of the contiguous light beams in the wavefront are determined by the computer. Next, these actual x-y positions are compared with the corresponding idealized x-y positions of contiguous light beams in a plane wavefront. These comparisons are then used to determine any deviation there may be between the actual x-y position of a light beam""s focal point and a corresponding idealized x-y position for the focal point. Using the deviations for each of the light beam focal points, the actual wavefront that is characteristic of the detected wavefront can be analyzed, reconstructed or otherwise used as desired.